Lecithin cholesterol acyltransferase (LCAT), the major enzyme which esterifies cholesterol present in plasma lipoproteins, plays a central role in HDL metabolism. Patients with LCAT deficiency may present with corneal opacities and renal disease as well as reduced plasma HDL-C and apoA-I concentrations and increased triglycerides. To evaluate the role that LCAT plays in reverse cholesterol transport and the development of atherosclerosis we have established a mouse model for human LCAT-deficiency by performing targeted disruption of the LCAT gene in mouse ES cells. Homozygous LCAT-deficient mice were viable and healthy at birth. Plasma LCAT activity in age-matched control siblings (n=38, LCAT act=42 plus/minus 5 nmol/h/ml) was decreased to 29 plus/minus 3 noml/h/ml (p<0.01) in heterozygous LCAT deficient mice and to <0.7 nmol/h/ml in homozygotes. Compared to control mice, heterozygous LCAT-deficient mice had decreased (% of normal, p<0.01); plasma cholesterol (76%), cholesteryl ester (74%), phospholipids (87%), HDL-cholesterol (77%) and apoA-I (73%) and increased (% of normal, p<0.01); triglycerides (130%). Homozygous LCAT-deficient mice had a decreased in cholesterol (28%), cholesteryl ester (14%), phospholipids (46%), HDL-cholesterol (3%) and apoA-I (17%). Analysis of plasma lipoproteins in homozygous LCAT-deficient mice by FPLC demonstrated severe reduction in HDL-cholesterol with the presence of smaller sized particles, as well as triglyceride-rich IDL/LDL. No evidence of corneal opacities or renal disease has been detected in 2 month old animals. The availability of a homozygous animal model for human LCAT-deficiency will facilitate our understanding of the role that LCAT plays in the development of renal disease and atherosclerosis as well as permit the evaluation of the feasibility of performing gene transfer in human LCAT deficiency states.